1. Field of the Invention
The invention relates to refractory shapes in general and, more particularly, to an improved refractory brick which exhibits resistance to temperature change and good pressure flexibility. The refractory brick comprises sintered magnesium oxide and MgO.Al.sub.2 O.sub.3 spinel, wherein the magnesium oxide content is between about 82 to 90 percent by weight and the aluminum oxide content is between about 10 to 18 percent by weight, and a content of up to about 3 percent by weight of customary impurities.
2. Description of the Prior Art
While pure magnesium oxide bricks were previously used, specifically for lining furnaces in the cement industry, over the past several decades it has been a long-standing goal in the art to develop refractory construction materials for open-hearth furnaces and to produce high-quality basic bricks, specifically with improved resistance to temperature changes and slag, and with excellent refractory qualities. The results of this on-going search have included basic bricks containing chromium and sintered dolomite products.
In the beginning of the 1970's, further improvements were made with magnesite-chrome bricks by the development of the high-temperature firing technology, specifically with regard to the physical-ceramic properties of these bricks.
One example of such bricks can be had from U.S. Pat. No. 4,039,343 issued to Stein, et al. on Aug. 2, 1977, entitled "Improved Performance Direct Bonded Basic Refractory Brick And Method Of Manufacture", the contents of which are incorporated herein by reference. This patent teaches a refractory shape having about 40 to about 75 percent by weight high purity magnesite, about 25 to about 60 percent by weight chrome ore, and about 0.5 to about 10 percent by weight chromic oxide powder. Such dolomite and magnesite-chrome bricks are used in the cement industry, particularly in stable, supported areas of the sinter zone of a rotary hearth furnace.
However, the problems have not been solved with respect to the reduction of premature wear in these zones, in which stable conditions do not prevail. Such zones include those areas prior to and immediately after the primary sintering zone. With magnesite-chrome bricks, moreover, the hexavalent chromium represents a pollution problem. A variety of techniques have been employed to improve the wear characteristics of the lining in steel-making furnaces and the like. For example, U.S. Pat. No. 4,468,780, issued to Schellberg, et al. on Aug. 28, 1984, entitled "Method Of Lining A Steel-Making Furnace", the contents of which are incorporated herein by reference, teaches a method of lining a steel-making furnace with bricks to form a first lining and at least a portion of a wall area of a furnace with bricks and/or a water cooling box to form a second lining. An example of the use of dolomite bricks and bauxite bricks as a refractory fireproof inner liner in a molten iron-containing vessel can be had from U.S. Pat. No. 4,432,536, issued to Coordes, et al. on Feb. 21, 1984, entitled "Molten Iron Containing Vessel With Improved Refractory Lining", the contents of which are incorporated herein by reference. Further developments led to the creation of basic refractory bricks, which essentially consist of sintered magnesium oxide as well as MgO.Al.sub.2 O.sub.3 spinel (so-called genuine spinel). For applications in the cement industry, a pre-synthesized spinel provides a particularly advantageous constituent for the manufacture of bricks. The material characteristics of such magnesium oxide spinel bricks can offer significant advantages over conventional magnesite chrome products. Thus, for example, resistance to alkalis as well as higher degrees of refractoriness can be achieved. Practical experience has shown that, to some extent, and particularly under changing conditions, the mechanical properties of these bricks are not sufficient to withstand the alternating stresses which occur and which cause the formation of cracks and premature wear of the bricks. An example of such spinel magnesia bricks can be had in U.S. Pat. No. 4,389,492, issued to Tanemura on June 21, 1983, entitled "Spinel-Magnesia Basic Brick", the contents of which are incorporated herein by reference.
In order to improve the resistance to wear of the lining of rotary hearth furnaces used in the cement industry, it has been suggested, as in German Laid Open patent application No. DE-OS No. 30 23 229, to use a compound lining in order to compensate for the severe strains incurred by such furnaces. Such compound linings typically comprise the use of magnesium oxide or magnesite-chrome bricks with so-called pyroplastic properties, which are laid in combination with so-called hard core bricks. For example, the two grades of brick can be laid in a checkerboard pattern, whereby the magnesium oxide or magnesium oxide chromium bricks form a matrix, between which the refractory hard core bricks are inserted as appropriate. However, such a lining does not lead to a uniform reduction of stresses. The bricks tend to work their way out of position, and the so-called pyroplastic bricks have only relatively low refractory characteristics. The use of a checkerboard pattern of bricks also requires that an inventory of at least two different refractory grades must be maintained. This obviously results in a significantly higher cost for the lining itself, as well as complications in both the repair of the damaged lining and the maintenance of a proper inventory.
Refractory bricks which are subjected to severe variations in temperature and which must simultaneously exhibit high resistance to infiltration and chemical corrosion must therefore exhibit suitable characteristics with respect to gas permeability (GD), pore volume , refractoriness under load (DFB) and the compression settling (DFL).
Heretofore, under those techniques conventionally utilized, some of the above-mentioned characteristics have been met by the available refractory bricks. However, a combination of all four characteristics has not yet been achieved in a single refractory brick. Table I shows values for the above-mentioned prior art characteristics for bricks. In Table I, MA represents magnesium oxide spinel brick and MC represents magnesite-chrome brick.
TABLE I ______________________________________ MA 1 MA 2 MC 1 MC 2 ______________________________________ P.sub.v percent 19 17.5 18 16.5 GD Nanoperm 5 4 15 20 DFB ta .degree.C. 1550 &gt;1740 1600 &gt;1740 DFL percent compression -5 -1 0.5 0.3 settling at 1400.degree. C. ______________________________________